Rocky Mountain Spotted Fever in Dogs

Rocky Mountain spotted fever (RMSF) is a disease of humans and dogs caused by Rickettsia rickettsii. R rickettsii and closely related members of the spotted fever group of rickettsiae exist in parts of North, South, and Central America. These pathogens are transmitted primarily through the bites of infected ticks.

In the US, the American dog tick (Dermacentor variabilis) and the Rocky Mountain wood tick (Dermacentor andersoni) are the primary vectors for R rickettsii. The incidence of RMSF follows the vectors' distribution; the southeastern and south-central regions of the US have the highest reported number of cases.

In South America, several Amblyomma spp of ticks have been implicated in RMSF transmission. The brown dog tick (Rhipicephalus sanguineus) is the primary vector in some focal areas of Arizona (particularly on native American tribal lands) and Mexico.

Transmission of RMSF typically occurs within hours after the tick attaches to a host. Although serological studies have reported seropositivity rates of up to 70% in areas with recent history of an outbreak, seroprevalence in the southern US is reported to rarely exceed 10% (1, 2).

Because of their susceptibility to R rickettsii and relatively higher rates of tick exposure, dogs can serve as sentinels of risk for R rickettsii infection in humans; however, direct transmission from dogs to humans has not been documented.

Clusters of RMSF cases are frequently reported in defined geographical areas, and temporally associated infections can occur both in dogs and in their owners.

Dogs are highly susceptible to clinical infection with R rickettsii. In contrast, R rickettsii infection is rarely diagnosed in cats.

Early signs of Rocky Mountain spotted fever in dogs include the following:

• fever up to 40.5°C (105°F)

• petechiae

• lymphadenopathy

• polyarthritis

• respiratory signs (coughing, tachypnea, epistaxis)

• ocular signs (uveitis, retinal hemorrhage, conjunctivitis)

• edema of the face, scrotum, or extremities

• neurological signs (ataxia, vestibular signs, paresis, seizures)

Findings of R rickettsii infection in CBCs, serum chemical panels, and urinalyses are typically a consequence of the severe vasculitis caused by these infections. Thrombocytopenia, secondary to consumptive as well as destructive processes, is the most common finding. Proteinuria, thought to be secondary to glomerular damage, results in hypoalbuminemia.

• Clinical evaluation

• Serological testing

• PCR assay

Dogs presenting with characteristic clinical signs of Rocky Mountain spotted fever, particularly fever or neurological abnormalities, should be assessed for RMSF, and it is crucial to maintain an index of suspicion in areas where the tick vectors can be found.

Differential diagnoses of RMSF include other causes of fever of unknown origin. In areas where the brown dog tick is the main vector, most febrile dogs will likely have Ehrlichia canis infection, which is more common than RMSF.

Very early in R rickettsii infection, PCR assay of circulating blood might be positive; however, many severely affected animals become PCR-negative as the bacteria become resident in endothelium. PCR assay of a tick-bite eschar can be useful when such a lesion is present.

Suspicious cases should be treated with appropriate antimicrobials before tests confirm an RMSF diagnosis. The therapeutic response is usually dramatic, as it is with other canine rickettsial diseases.

The indirect fluorescent antibody test (IFAT) and ELISA are available for serological testing. However, there is a high incidence of cross-reacting antibodies against a variety of nonpathogenic and less-pathogenic rickettsiae of the spotted fever group, so a single seropositive test does not confirm RMSF as a cause of disease. Dogs that have been sick for a week or more might have very high titers, which can support a diagnosis; however, antibodies might persist for months after acute RMSF infection.

Given the acute nature of RMSF infections, use of paired or convalescent titers, with the second serum sample obtained within 1–3 weeks after the first sample, are vital to confirm infection. Demonstration of a 4-fold rise in titer can retrospectively support a diagnosis of RMSF.

• Doxycycline

• Supportive care

When Rocky Mountain spotted fever is suspected, antimicrobial treatment should be initiated before confirmation by serological testing, because delayed administration of antimicrobials results in higher morbidity and mortality rates.

Doxycycline (5 mg/kg, PO or IV, every 12 hours; or 10 mg/kg, PO or IV, every 24 hours) is the treatment of choice for RMSF, regardless of the dog's age. Although treatment for only 7 days is adequate in most cases, prolonged treatment (14–28 days) is recommended when coinfection with another vector-borne agent is suspected.

Chloramphenicol was used to treat RMSF in the past; in humans, however, its use is associated with higher rates of fatal outcome due to idiosyncratic aplastic anemia, and it is not recommended. Enrofloxacin (3 mg/kg, PO, every 12 hours for 7 days) has been shown to be effective in clearing infection in experimentally infected dogs.

Supportive care for dehydration and hemorrhagic diathesis might be necessary in cases of RMSF. Animals with neurological dysfunction might have residual deficits. Immunity appears to be lifelong after natural infection; therefore, recurrent episodes should not be attributed to RMSF.

Precautions should be taken for safe removal and control of ticks. When R rickettsii transmission from Rhipicephalus sanguineus is suspected, it is important to use medications and products with proven efficacy against this tick species.

Because R sanguineus infestations can be problematic in kennels and around homes, and because long-term tick control is needed for outbreak control, the use of effective long-acting tick collars on all susceptible dogs should be considered in endemic areas. Collars containing fipronil, isoxazoline, amitraz, or pyrethroids have proven activity against R sanguineus.

R rickettsii is considered a zoonotic pathogen. The potential for household clustering and large urban outbreaks, particularly in areas with transmission by brown dog ticks, makes RMSF a disease of significant public health concern.

Although RMSF occurs both in animals and in humans, the involvement of a required intermediate tick vector for transmission means that dogs and other infected animals do not pose a direct transmission risk to humans in normal circumstances. Nevertheless, dogs are the main host for brown dog ticks, and exposure to an increased number of dogs can increase overall risk for other dogs and humans.

Particularly in areas where RMSF is transmitted via R sanguineus, close cooperation between veterinary, medical, and public health officials is important to achieve control.

• Rocky Mountain Spotted Fever. Companion Animal Parasite Council.

• Kidd L. Emerging spotted fever rickettsioses in the United States. Vet Clin North Am Small Anim Pract. 2022;52:1305-1317.

• Also see pet owner content regarding Rocky Mountain spotted fever in dogs.

1. Demma LJ, Traeger M, Blau D, et al. Serologic evidence for exposure to Rickettsia rickettsii in eastern Arizona and recent emergence of Rocky Mountain spotted fever in this region. Vector Borne Zoonotic Dis. 2006;6(4):423-429. doi:10.1089/vbz.2006.6.423

2. Pieracci EG, de la Rosa JDP, Rubio DL, et al Seroprevalence of spotted fever group rickettsiae in canines along the United States-Mexico border. Zoonoses Public Health. 2019;66(8):918-926. doi:10.1111/zph.12642